The invention concerns a transmission method in accordance with the precharacterizing clause of claim 1, as well as a transmitter-receiver arrangement for execution of the method in accordance with the precharacterizing clause of claim 7.
In the case of known transmission methods, the communication to be transmitted is modulated in the transmitter onto a high frequency carrier signal and forwarded over the transmission distance to the receiver, which, for recovery of the communication displays an appropriate demodulator. There exists comprehensive literature concerning modulation of analog signals. Modern communication methods use digital or digitized data, since these types of signals can be processed rapidly and in cost-effective fashion by means of signal path processor technology, with the means available today, even in the case of a large amount of information.
If the communication signal to be transmitted is present in digitized form as a bit sequencexe2x80x94as is the case in modern mobile radio networksxe2x80x94then modulation is accomplished by changing the frequency, the phase or the amplitude of the carrier signal as a function of the particular value of the information of the bit series to be transmitted. As concerns digital modulation of carrier signals, known from COUCH, L. W.: Digital and Analog Communication Systems, 4th Edition, Macmillan Publishing Company (1993) are various digital modulation methods, for example Amplitude Shift Keying (ASK), Two-Phase Shift Keying (2-PSK), Two-Frequency Shift Keying (2-FSK), or more recent methods such as the spread spectrum modulation method. In each case, demodulation takes place in the receiver corresponding to the modulation method used at the transmitter, and therewith recovery of the digital communication signal as bit sequence in the form of pulses following one after the other. A known modulation method of communication technology, as mentioned, provides angle modulation as the generic term for frequency or phase modulation. In the case of the known methods, this type of modulation, however, serves exclusively for the purpose of superimposing the communication onto a carrier.
With all the methods of this type, there exists the disadvantage that the quality of the communication signal recovered at the receiver decreases strongly with the distance between receiver and transmitter, with disturbance elements over the transmission distance.
In order to achieve a prescribed protection against disturbances over a desired range in the case of transmission of a communication over a disturbance-encumbered transmission distance, transmitter power may not drop below a predetermined value.
First, the therewith required high transmitting power has the disadvantage that the radiated power during transmitting operation is correspondingly high, which, in particular in the case of battery-operated devices, as with mobile telephones, is disturbing because of rapid battery depletion. Secondly, there exist fears that the electromagnetic radiation emanating from the transmitter can lead to damage to the human body. This is to be taken into consideration in particular in the case of mobile telephones because of the comparatively short distance to the user.
The task underlying the invention is to obtain a transmission method of the initially mentioned type, or to be precise, a receiver-transmitter arrangement for execution of the method, which, with in general at least transmission quality remaining the same, enables a reduction in transmitting power or an increase in range.
This task, starting out from a method in accordance with the precharacterizing clause of claim 1, is resolved by its characterizing features, or relative to the arrangement for execution of the method, by the features of claim 7.
The invention includes the technical teaching to transmit xe2x80x9cfolded (doubled) pulsesxe2x80x9d between transmitter and receiver, which are specially designed pulses that are defined in more detail later. These folded pulses, because of their particular characteristics, can be used not only for raising the amplitude by appropriate compression methods with correspondingly suited dispersion filters, but also because of their particular, highly correlative properties can be used for additional correlative and auto-correlative suppression of noise compared to the signal. The particular modulation and the special composition of these transmitting elements, called here xe2x80x9cfolded pulsesxe2x80x9d, permit raising the signal-to-noise ratio in the analog signal processing at the receiver. In this way there can be achieved through an improvement of the signal-to-noise ratio in the receiver a reduction of transmitting power or an increase in range or alternatively a reduction of the error rate.
To be understood here under the concept xe2x80x9cfolded pulsexe2x80x9d, and in what follows, is the superposition of at least two opposed, angle-modulated pulses (components) having essentially the same durationxe2x80x94in their basic form also designated as xe2x80x9cchirp signalsxe2x80x9d, whereby the angle modulation of the two pulses results from the fact that the frequency of the one component changes, in the mathematical sense increasing monotonically during the duration of the pulsexe2x80x94and in the second pulse component decreasing monotonically. The folded pulse is, therefore, to be defined as that it simultaneously consists of at least two angle modulated pulses (chirp signals) at a frequency varying counter to each other, with the relative phase position of the components to each other also capable of further being used for differentiating these types of signals.
For a better understanding of chirp signals, the components of the xe2x80x9cfolded pulsesxe2x80x9d, let first their basic properties and next the special, advantageous properties of the folded pulses be dealt with.
An angle modulated pulse of a certain time duration xcex94t having a certain frequency deviation xcex94f is, among other things, capable of being characterized by its time-bandwidth product xcexa8=xcex94txc2x7xcex94f. Through special, so-called xe2x80x9cdispersion filtersxe2x80x9d that are four-polar with a defined differential delay time behavior, it is possible to push together, that is to say, compress such angle-modulated pulses along the time axis, in the receiver. The energy of the original pulse of duration xcex94t [s] having the amplitude U0[V] across the resistance R1[xcexa9], which is given by the expression (U02xc2x7xcex94t)/R1, is retained in compression, in the first instance, as assumed loss-free dispersion. Accordingly, for the shorter compressed pulse of duration xcex4 one can estimate the energy with (Û2xc2x7xcex4)R1, where Û[V] represents the increased pulse amplitude resulting from compression.
Hence one obtains
(U02xc2x7xcex94t)/R1=(Û2xc2x7xcex4)/R1 
Accordingly, the ratio of the squares of the voltages is equal to the inverse ratio of the times between the originally transmitted pulse of duration xcex94t and the average duration xcex4 of the compressed pulse, applicable, therefore
Û2=U02xc2x7xcex94t/xcex4=U02xc2x7xcex94txc2x7xcex94f=U02xc2x7xcexa8,
where xcex4=1/xcex94f. Accordingly, the voltage in the receiver is raised by compression by a factor that corresponds directly to the root of the time/bandwidth product.
Hence, the chirp pulse, when it is compressed through dispersion filters, produces a first improvement of the signal-to-noise ratio. Because the signal-to-noise ratio xcfx81[dB] is defined by 20times the logarithm of the ratio of the signal voltage Û[V] to the noise voltage Ur[V], With the above equations on obtains:
xcfx81=20log(Û/Ur=10log((U02/Ur2)"psgr")=10log(U02/Ur2)+10log"psgr"
whereby can be seen that the S/N ratio xcfx81 is improved by the portion +10log"psgr".
These relationships are known and at the present time are used, for other reasons, only in radar technology and for transmission of signals in optical lines, however not for general communications transmission. However, the chirp signals have yet another property, not used until now, that permits a second improvement of the S/N ratio.
By multiple correlation of several chirp signals, capable of being achieved in the form of the folded pulses is an automatic correlation in the receiver that causes a further, additional, very impressive S/N gain, over that capable of being achieved by the compression of the type represented above by, for example, multiplication of folded pulses.
This relies on the possibility of obtaining combinations of such chirp pulses in the form of folded pulses, which, with the use of dispersive filter arrangements, makes it possible to time shift by means of the delay properties of the filters the original components running differently on the time axis, such that coincident signals can be generated in such a manner that this time shift can be used for correlation of the signals.
In this manner, the folded signals can be characterized as highly correlated communication signals which, for several reasons, can be ideally used for communications transmission. Indeed, in the first instance, because of their length and bandwidth they cause a loss in bit rate, however allow at the other end, here the receiver end, a clear gain in noise reduction of the most varied of disturbing elements, including white noise, therefore unavoidable thermal noise.
They are highly correlated because several physical conventions between transmitter and receiver must be met, and the dispersive filters must also be matched to the phase characteristic of the transmitted folded pulse in the receiver. These are:
1. the frequency position of the carrier frequency (middle frequency),
2. the bandwidth of the frequency of the angle modulated pulses (frequency deviation),
3. the angle modulation time characteristic of the transmitting pulse components,
4. the time duration of the folded pulse,
5. the direction of the angle modulation (monotonically increasing or decreasing frequency with time) and their overlap,
6. the phase position at a prescribed point in time within the time duration of the angle modulated pulse, and the phase position of the components relative to one another, and
7. the amplitude of the angle modulated pulse.
These parameters between transmitter and receiver can be freely agreed upon, up to the seventh one, in order to serve as information carriers with correspondingly embodied receivers.
They permit a wide variation in the parameters beneficial to information transmission. The transmission speed, hence the bit rate, used to be the most important goal of the configuration of transmission systems. Actually, in the first instance it is lowered by the time duration of lengthened pulses. Channel capacity can be increased by operating, through time or frequency multiplexing methods, different channels independently at different times or at different frequencies or with different frequency-time modulation characteristics and different folded pulse combinations. The bit rate per channel times the number of possible channels only then yields the total amount of transmittable information content per unit of time.
Therefore, the variability of the above parameters, which extend over the time and frequency position, allows an additional gain, when the above mentioned variables are agreed upon in different conventions between transmitter and receiver.
These deliberations show that chirp pulses, especially folded pulses, can almost be perceived as a special xe2x80x9ccarrier substratexe2x80x9d for transmission of actual communications. This modulation, therefore, occurs independently of the modulation provided for the communication, which should be, as much as possible, orthogonal to the first. The modulation, to be characterized here for the sake of clarity as carrier substrate modulation, therefore produces an additional relationship or correlation between transmitter and receiver, and additionally serves to eliminate noise, primarily thermal noise and also other disturbing elements, because these latter can not display this added modulation.
The transmission method represented here for transmitting communications signals between a transmitter and a receiver over a disturbance-burdened transmission distance represents a combination of one of the known types of pulse modulation or types of pulse code modulation, and a special angle modulation acting orthogonal to the first type of modulation, with the base-band signals of the communication capable of being generated, for example, in Pulse Position Modulation (PPM) or Pulse Code Modulation (PCB) or Pulse Amplitude Modulation (PAM) or in Differential Pulse Code Modulation (DPCM) or as Pulse Delta Modulation or Spread Spectrum modulation, or one of the known modifications of these types, in a manner corresponding to the prior art.
These communication-related modulations of the analog or digital signals, however, are here modulated onto a carrier oscillation, which is not generated in the transmitter device during the duration of the pulse as usual from a carrier frequency that is constant in frequency, but rather the carrier frequency is additionally multiply angle modulated such that, for one thing, the angle modulation components that are reverse to one another in the case of the folded pulse, and for another thing the amplitude change as signal information or the pulse interval values (in the case of PPM) of the angle modulated carrier, are executed simultaneously and for a different purpose, as a combination of modulation types that are independent of one another, so-called xe2x80x9cmodulation types orthogonal to one anotherxe2x80x9d, with the known types of modulation serving for transmitting the communication and, additionally, the angle modulation combinations in the special form of folded pulses being used as signals that are capable of being highly correlated using dispersive filter arrangements for correlative noise suppression.
The sequence of such folded pulses is transmitted to the receiver over the transmission distance that is generally disturbed by interference from other transmitters and by white noise. The concept xe2x80x9ctransmission distancexe2x80x9d is to understood here as being general, and includes wireless transmission distances where information transmission from transmitter to receiver is accomplished by means of electromagnetic waves, as well as line-connected transmission distances where transmitter and receiver are preferably connected together over light wave conductors, coaxial cable or simple electrical lines.
Moreover, the invention can be used in the case of data storage, for example on magnetic data carriers. In this case, the transmitter is designed as a writing unit that writes the data on the data carrier, while the receiver is constructed as a reading unit that reads out the stored data from the data carrier.
The receiver can demodulate the two modulation types that are orthogonal to one another, with the receiver, for this purpose, in the progression of its block diagram, displaying between antenna and rectifier dispersive filters of the type defined in accordance with the invention, where such filter arrangements are indicated that serve for automatic signal-to-noise improvement through the correlative properties of the folded pulses, and are simultaneously capable of still raising these signals in their amplitude by compression.
Since the chirp signals contained in the folded pulses permit a gain in the signal-to-noise ratio through compressibility of the signal amplitude, and the dispersion filters can be arranged such that their properties that are inverse to each other generate two mirror-symmetrical output signals from the chirp signal components, these simultaneous, correlated pulses can be added, multiplied or subtracted, cut out or suppressed, and in this way offer a quasi-autocorrelative emphasis of signal versus noise.
Another very critical consideration can be derived from the circumstance that the rise time of the compressed pulse corresponds to the full bandwidth of the chirp signal and, in its time position, is defined very exactly within a receiving arrangement. Consequently, this transmission method is plainly predestined for pulse position modulation (PPM). Even if one would always send out two chirp signals, the first of which served as the time reference for the interval to the second pulse following it, the total duration would be only 2.5 times the pulse duration. A signal of this kind can be used for analog signal transmission, but also for transmission of digital signals, to the extent, however, that the increased channel capacity through the increased bandwidth is likewise used.
The dispersive filter arrangements, as they will be presented later in examples of applications, can simultaneously fulfill several functions, and therewith reduce the needed expenditure in possible receiver structures.
First, they cause a increase of the signal versus noise by mere compression of the folded pulse components.
Second, these arrangements simultaneously enable the folded pulse components, through appropriate arrangements of the filters, to lead to coincident mirror-symmetrical signals, which, by automatic correlation, lead to a further gain relative to the S/N ratio.
Third, added in the case of a multiplication of the coincident and compressed signals with autocorrelative multiplication of signals of like frequency position (mirror-symmetrical frequency position) is that, without further filters an automatic, multiplicative and coherent demodulation of the compressed signal will be effected, which otherwise could only be achieved with expensive PLL circuits.
If in the receiver one controls the folded pulse, as it was defined initially, across two parallel connected dispersion filters with reversed complementary dispersion, two mirror-symmetrical signals arise at both outputs of these filters.
In the case of angle modulated folded pulses, the two dispersion filters have two characteristic curves acting inversely to each other. While the phase response over the frequency is, in each case, parabolic, the group delay time derived therefrom over time is a straight line which also rises with rising frequency, while the other filter acts in complementary fashion to the characteristic of the group delay time, therefore the group delay time becomes greater with rising frequency.
Hence, in the case of linear frequency modulated pulses, the group delay time characteristic is a straight line. In the case of non-linear frequency modulation, group delay time of the dispersive filter at any one time represents the internal function concerning the modulation characteristic at that time. Therefore, in the case of complementary, non-linearly modulated folded signal components, the demodulated dispersion filters display corresponding complementary group delay time characteristics.
Since the superposed portions of the folded pulse consist of two components, and these two portions are connected to two inversely-acting, parallel-connected dispersion filters, four processes take place at the same time:
In the case of the component that displays a frequency increasing with time (positive frequency pattern), the higher frequency portions are delayed relative to the frequency by one of the two parallel-connected filters having a negative group delay time characteristic. By this means, the originally positively chirped signals are compressed, with the counter-running, negatively chirped folded pulse components being expanded with respect to time to double the duration of the initial pulse.
The other filter acts in reverse, because it delays the lower frequencies more strongly than the high frequencies (positive group delay time characteristic), whereby the component running from high frequencies to low frequencies is compressed, and the pulse component running from low frequencies to high frequencies is expanded to double the duration of the initial pulse.
Hence, the two dispersion filters, in each instance with one of the two angle-modulated pulses forming in their superposition the folded pulse, lead to a time compression with an increase in amplitude corresponding thereto, whereas the other pulse portion is expanded to double the duration, which leads to a corresponding decrease in amplitude.
Since the noise at the input is not correlated in comparison to a signal of that type, but rather, because of the dispersion properties of the dispersive filters was not changed in the same way, the noise signal is uncorrelated to the signal at the output of the two filters.
Therewith, in the analog section of the receiver, by means of analog signal processing, one can make use of certain principles that can be used for noise suppression, and actually to a large extent independently of one another, as simulations have shown.
For practical implementation of the dispersion filters required by the system, serving this purpose nowadays, based on the prior art, are preferably Surface Acoustic Wave (SAW) filters, since these types of filters can be manufactured with high reproduction accuracy and stability. Moreover, these types of surface acoustic wave filters offer the advantage that amplitude response and phase response can be dimensioned independently of one another, which opens up the opportunity of developing in a single unit the narrow bandpass filter and the dispersion filter needed in each receiver. The configuration of the dispersion filter as a SAW filter module further advantageously enables the integration of several dispersion filters together with low pass filtering, addition and subtraction on a substrate, so that it can become possible to obtain a compact SAW element as the core of the arrangement in accordance with the invention.
Therefore, a SAW filter element on a substrate is preferred, the SAW filter comprising two parallel dispersive filters acting inversely to each other, having two inputs and outputs and each with additional outputs for the sum and difference of the output signals. These functions could all be installed on a single substrate. The normally differential inputs and outputs were represented here, for the sake of simplicity for block diagrams, only by a line.
The transmission method in accordance with the invention, relative to the angle modulation proposed for the transmitter end is, as can be seen, not limited to a linear frequency change during the duration of the pulse. It is critical that the delay time characteristic of the dispersion filters provided on the receiver end be adapted to the angle modulation of the two pulses forming the folded pulse by their superposition at the transmitter end in such a way that there appear at the output the dispersion filters arranged at the receiver end a combined signal that consists of a time-compressed pulse with correspondingly increased amplitude and a time-expanded pulse with correspondingly reduced amplitude.
These two combined signals can now be either added, subtracted or multiplied and, as shown, will hereby, or by suppression or by cutting out the coincident portion, be used for improvement of the S/N ratio in the receiver.
The addition of the combined signals yields a superposition of the compressed signal portions, as well as an superimposing of the doubly-expanded chirp signals, and an addition of the noise arriving along the transmission path. Since the compressed signals reach the addition stage coincidently, their signal amplitudes are added, hence doubled. Accordingly the signal is increased by 6 dB. The noise, however, which is not correlated and whose phase and amplitude vary, can only be added relative to the power. Therefore, its amplitude increases statistically only to 3 dB. Accordingly, there results an average signal-to-noise improvement of 3 dB, because, in the case of the signal, based on the simultaneous occurrence, the voltages are added, and in the case of noise, based on the statistical occurrence only the powers are added. The coincidence of the mirror-symmetric compressed components at the output of the dispersion filters arranged at the receiver end therefore leads to an S/N gain only for a folded signal in the event of its summation.
Subtraction of the combined signals, depending upon the phase position of the folded components of the folded signal relative to one another, leads to an improvement of the signal-to-noise ratio. Depending upon phase position of the signals, addition and subtraction are merely complementary processes to one another.
In the case of multiplication of the two combination signals arising at the output through the parallel-connected dispersion filters, similar conditions arise, as known from autocorellation.
In the case of the known autocorrelation method, periodic or quasi-periodic signals are offset through a delay line for about the duration of the period and are multiplied with the arriving signal that is not conducted over a delay line. The uniformity of a signal after a duration of one period leads to squaring of the then coincident amplitudes. The noise, however, while not capable of being correlated by the delay line, is hereby suppressed, as is well known. Autocorrelation belongs to the most efficientxe2x80x94although not linearxe2x80x94method for bringing into prominence periodic or quasi-periodic signals as compared to noise, therefore to raise the signal/noise interval.
The same physical effect can very advantageously be achieved for the folded signal. Since the folded signal is composed in such a way that it generates two combined and coincident output signals that are symmetrical to one another, by means of two parallel-connected dispersion filters having a dispersion direction inverse to one another, said output signals being characterized by the fact that located in their time center, in both branches, are compressed signal portions that are magnified. Multiplication of these magnified signals that are compressed over a narrow time range leads to squaring of the signal amplitudes.
The noise, however, is not correlated and besides was expanded in its time pattern, through the dispersive filters, hence also lowered in its amplitude. Multiplication of the noise portions therefore leads, relative to the squared signal, to a very much lower amplitude.
Accordingly, a similar physical effect occurs here as in the case of the auto-correlation of the periodic signals, here with an aperiodic signal. Although the auto-correlation equation for folded signals would appear to be different than for periodic signals, since it is not the signals that will be offset by the duration of the period through one a delay line, but rather there being present two frequency-dependent delay lines having a dispersion direction the reverse of one another, which act reciprocally on the folded signal such that the compressed signals and the expanded signals appear coincidentally in a sort of timed mirror-symmetry, occasioned by the reciprocal multiplication is an impressive suppression of noise.
While normal autocorrelation presumes periodic or quasi-periodic signals, it is not usable on digital sequences, for example the pulse code modulation method. The folded signal, however, is a signal of a specific duration that does not repeat itself. Nevertheless, as was demonstrated, it is in itself automatically capable of being correlated.
Generation of the angle-modulated pulses, which in their superposition form in each case a folded pulse, can be accomplished, in accordance with the prior art, is several ways, of which some will be briefly described in the following.
In one variant of the invention, as an approximation, a Dirac pulse is first generated and fed to a low-pass filter whose filter characteristic displays a peak shortly before reaching the limit frequency, and therewith converts the Dirac pulse into an sinc pulse (split pulse), whose form is described by the known sinc function (x)=sinx/x. The sinc form output signal of the low-pass filter is next given over to an amplitude modulator, which impresses onto the carrier oscillation a sinc-form envelope. If the signal generated in this fashion is added to a parallel circuit of two dispersing filters having characteristic curves that are the reverse of each other, then appearing at the output of both filters are two angle-modulated chirp signals, through whose addition or subtraction arise two different folded pulses, that can be designated here as so-called xe2x80x9csum or difference signalsxe2x80x9dxe2x80x94both are folded pulses with a different phase position with respect to one another.
According to a preferred configuration of the invention, generation of the frequency modulated pulse, on the other hand, is by means of a PLL loop (PLL: Phase Locked Loop) and a Voltage Controlled Oscillator (VCO). The individual pulses of the input signal that are present in digital form are first converted for this purpose into sawtooth-shaped pulses through an integrator, whereby the direction of ascent of the individual pulses depends upon the amplitude of the input signal. The signal generated in this fashion is then used for control of the VCO, so that the frequency of an output pulse during the duration of the pulse increases or decreases linearly as a function of the level of the input signal. If two oppositely running chirp signals are generated simultaneously by a suitable circuit of this kind, then the folded signals can be generated either by addition or subtraction as sum or difference signals.
According to another advantageous embodiment of the invention, generation of the frequency modulated pulses in the transmitter is accomplished by a digital signal processing unit, which advantageously enables the realization of arbitrary frequency patterns during the duration of the pulse.
As a rule the information to be transmitted is present in digital form as a binary signal where in a simple variant of the invention, the impressing of this information upon the folded pulses takes place by the fact that only during a logical HIGH-level of the information-carrying input signal is a folded pulse transmitted, while a logical LOW-level of the input signal leads to a transmission pause. The reverse of this convention is also possible.
What is decisive in this variant of the invention is merely the fact that only one logical level of the information-carrying input signal is actively transmitted.
In contrast to this, in the preferred implementation form of the invention both a logical HIGH-level and a logical LOW-level of the information-carrying input signal are actively transmitted, which leads to an increased security against noise. For this purpose, on the transmitter end two different folded pulses are generated, depending on the then-existing binary value of the input signal.
Thus, it is advantageous to transmit during a HIGH-level of the information-carrying input signal a folded pulse that consists of the sum of two oppositely angle-modulated pulses. During a LOW level of the input signal, a folded pulse consisting of a subtraction of two oppositely angle-modulated pulses is then appropriately generated. Accordingly, these two different folded pulses are distinguished from each other by the relative phase position of the folded-pulse components in each case.
In addition, these signals are applicable for almost all modulation processes known hitherto. However, they are ideally suited for pulse position modulation (PPM), in which the reduction of the bit rate does not matter so much, because for this a maximum of only two pulses are required; with synchronous processes only one pulse is required.
Furthermore, it can be advantageous to actively transmit both logical LOW-levels and logical HIGH-levels of the information-carrying input signal through in each case one folded pulse, the position of the transmitted folded pulse being preset depending on the then-existing value of the information-carrying input signal.
In this variant of pulse position modulation the invention is not limited to a binary input signal that exhibits merely two different signal levels, but rather is also applicable generally to digital input signals where folded pulses of different positions can represent a multiple bit level corresponding to the possible number of different signal levels of the input signal.
The transmission method according to the invention is not, however, limited to the above-described exemplary modulation methods, but rather can be combined with a great number of modulation methods, which methods, and others, are described in the publications named in the introduction insofar as reference is made to the contents of these publications. Even the modern spread spectrum modulation methods can be provided with the angle-modulated carrier substrate, in order to here effect a reduction in the white-noise level, which until now was not possible.
Other advantageous further developments of the invention are characterized in the dependent claims or are presented in detail with the aid of the figures, together with the description of the preferred implementation examples of the invention.